Method for forming biocompatible components

ABSTRACT

A method for forming biocompatible components from a stock of substantially completely consolidated material. The method includes the step of forming an incompletely consolidated stock from a powder. The substantially completely consolidated stock is then formed from the incompletely consolidated stock. Finally, the substantially completely consolidated stock is then machined to form the biocompatible component.

This is a division of U.S. patent application Ser. No. 08/006,747, filedJan. 21, 1993, now U.S. Pat. No. 5,466,530.

BACKGROUND OF THE INVENTION

The present invention relates generally to biomedical plant devices, andmore particularly to a method for forming biocompatible components.

A natural joint in the human body such as a knee joint may undergodegenerative changes due to a variety of etiologies. When thesedegenerative changes become advanced and are irreversible, it mayultimately become necessary to replace the natural joint with aprosthetic joint. Such a prosthetic joint often includes severalbiocompatible components which are formed from high strength syntheticmaterials. These materials are not only able to accommodate the variousloading conditions that the prosthetic joint may encounter, but are alsobiocompatible with the human body. An example of such high strengthsynthetic materials is ultra-high molecular weight polyethylene which isoften used when there is relative movement between the adjacent metallicsurface of a prosthetic joint.

Biocompatible components which are made from ultra-high molecular weightpolyethylene are often formed using one of two different techniques. Inone technique, a relatively precise amount of polyethylene powder isplaced between two halves of a die which are then simultaneouslycompressed and heated. After the powder is densified using standardsintering techniques, the die is allowed to cool. The biocompatiblecomponent is then removed from the die and is sterilized in a mannerwell-known to those skilled in the art.

In the second technique, a substantially completely consolidatedpolyethylene stock is first formed and then the biocompatible componentis machined from the substantially completely consolidated stock.Several methods exist which may be used to form the substantiallycompletely consolidated stock. In one method, the substantiallycompletely consolidated stock is extruded by placing polyethylene powderin a cylindrical chamber having an opening of a particular shape at oneend of the chamber. A hydraulically operated piston located at the otherend of the cylinder is then used to compress the polyethylene powder.The force exerted by the piston on the polyethylene powder causes thepowder to compact. Heat is also applied to solidify the powder as itmoves through the cylinder. In another method for forming asubstantially completely consolidated stock, polyethylene powder isplaced between two flat plates which are compressed while heat isapplied. As this occurs, the polyethylene powder is densified so as toform the substantially completely consolidated stock.

While these two techniques for forming biocompatible components areeffective, they nevertheless have certain disadvantages. With respect tothe first technique described above, it will be appreciated that onlyone biocompatible component can be made at one time. Accordingly, thistechnique is relatively inefficient in terms of the amount of timerequired to make the biocompatible component. With respect to the secondtechnique in which the biocompatible component is formed from asubstantially completely consolidated stock, the resulting consolidatedstock may often require a stress relief operation or an annealingoperation prior to machining. In addition, when polyethylene stock isformed by heating polyethylene powder between two plates acting underpressure, the resulting may have density gradients or voids due to therelatively nonuniform pressure applied to the powder across the plates.

In addition, methods are also known for treating ultra-high molecularweight polyethylene prior to being machined into a biocompatiblecomponent. One such method is disclosed in U.S. Pat. No. 5,037,928.However, during the procedure described in this reference, thepolyethylene stock is placed under a sufficient pressure so as to inducepressure crystallization of the stock. This pressure crystallizationtends to cause increased susceptibility to wear. In addition, the use ofthis relatively high pressure required that relatively expensivepressure containment vessels be used. Furthermore, this method describesprocessing preformed polyethylene stock which often has unwanted densitygradients or voids as described above.

SUMMARY OF THE INVENTION

An advantage of the present invention is to provide a method for formingbiocompatible components using a multiple-step technique which canproduce biocompatible components relatively quickly at a reduced cost.

A further advantage of the present invention is to provide a method forforming biocompatible components which produces a stock of consolidatedultra-high molecular weight polyethylene which can be machined withoutbeing subjected to a stress relief or annealing operation.

Another advantage of the present invention is to provide a method forforming biocompatible components which does not substantially increasethe crystallization of the stock used to form the biocompatiblecomponent.

A further advantage of the present invention is to provide a method forforming biocompatible components which uses both a cold isostaticpressure treatment as well as a hot isostatic pressure treatment.

A further advantage of the present invention is to provide a method forforming biocompatible components which enhances the bonding between thecomposite materials from which the biocompatible component is made.

A further advantage of the present invention is to provide a method forforming biocompatible components which facilitates the adhesion of aporous metal coating.

In one form thereof, the present invention provides a method for formingbiocompatible components from a powder such as ultra-high molecularweight polyethylene. The method includes enclosing the powder in a firstcontainer and subjecting the first container to a cold isostaticpressure treatment which forms an incompletely consolidated stock fromthe powder. The incompletely consolidated stock is removed from thefirst container and is placed in a second container which is thenlocated within a hot isostatic press and is subjected to a hot isostaticpressure treatment. The hot isostatic press treatment forms therelatively completely consolidated stock from the incompletelyconsolidated stock. The relatively completely consolidated stock is thenmachined into a biocompatible component.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sagittal elevational view of a knee joint prosthesisincluding a biocompatible component in the form of a tibial bearingformed from ultra-high molecular weight polyethylene by the preferredembodiment of the present invention;

FIG. 2 is a cross-sectional view of a cold isostatic press of the typeused in accordance with the teachings of the preferred embodiment of thepresent invention;

FIG. 3 is a perspective view of the first container used with the coldisostatic press shown in FIG. 2 according to the preferred embodiment ofthe present invention;

FIG. 4 is a cross-sectional view of a hot isostatic press of the typeused in accordance with the teachings of the preferred embodiment of thepresent invention;

FIG. 5 of the second container used in conjunction with the hotisostatic press shown in FIG. 4 according to the preferred embodiment ofthe present invention;

FIG. 6 is a flow diagram illustrating the steps for forming abiocompatible component according to the preferred embodiment of thepresent invention;

FIG. 7 is a perspective view of the container which is used inaccordance with the preferred embodiment of the present invention toenhance binding of the layers of composite material of a biocompatiblecomponent;

FIG. 8 is a perspective view of the container used in accordance withthe preferred embodiment of the present invention to reduce voids in astock of biocompatible composite material;

FIG. 9 is a perspective view of the container used in accordance withthe preferred embodiment of the present invention to enhance adhesion ofa porous coating to a biocompatible component; and

FIG. 10 is a perspective view of the container used in accordance withthe preferred embodiment of the present invention to facilitate adhesionof porous coated pads on a biocompatible component.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

It should be understood that while this invention is described inconnection with a particular example thereof, the scope of the inventionneed not be so limited. Rather, those skilled in the art will appreciatethat the following teachings can be used in a much wider variety ofapplications than the examples specifically mentioned herein.

Referring now to FIG. 1, a knee joint prosthesis is shown which isgenerally designated by the numeral 10. The knee joint prosthesis 10 isfunctionally depicted as being secured to a tibia 12 and a femur 14 of asurgically resected knee joint, with the tibia 12 and femur 14 beingshown in phantom. The knee joint prosthesis 10 is shown to include afemoral component 16 having a bearing surface 18. The femoral component16 is secured to the femur 14 by means of an inferiorly extendingfemoral stem 20 inserted into a matching bore created within the femur14 in a manner well-known to those skilled in the art.

The knee joint prosthesis 10 is further shown to include a tibialcomponent 22 that is secured to the tibia 12 by means of an interiorlyextending tibial stem 24 inserted into a matching bore created withinthe tibia 12 in a similar manner as that described above. The tibialcomponent 22 includes a platform-like tibial tray 26 which is used tosupport a tibial bearing 28 constructed by the method of the presentinvention. The tibial bearing 28 is formed to be symmetrically orientedabout the sagittal plane. In operation, the tibial bearing 28 provides abearing surface 30 that is operable to accept a rotatable, low frictioncontact relationship with the bearing surface 18 of the femoralcomponent 16.

The tibial bearing 28 is formed of a low friction material havingenhanced wear resistance properties. In a preferred embodiment, thetibial bearing 28 is machined from a substantially completelyconsolidated stock that is molded from an ultra-high molecular weightpolyethylene powder having a molecular weight of from about 3 million toabout 6 million. The ultra-high molecular weight polyethylene powder maybe any powder conforming to ASTM F-648, though preferable powdersinclude Hifax 1900 resin available from Himont and GUR 405 or 415 resinavailable from Hoechst Celanese. It will be understood, however, thatother suitable materials may be used to form a stock from which a tibialbearing 28 may be machined. For example, the formation of articles fromthis method can be accomplished using other polymer materials in powderform, preferably having a molecular weight of from about 3 million toabout 6 million. The specific method used to form the tibial bearing 28includes several steps which are more fully described below. However,several of these steps involve the use of either a cold isostatic pressor a hot isostatic press. Accordingly, the structure and operation ofthe cold isostatic press and the hot isostatic press will now bedescribed.

Referring to FIG. 2, a cold isostatic press 32 according to thepreferred embodiment of the present invention is shown which includes apressure chamber 34 that has an upper cover 36. The upper cover 36includes a threaded closure 38 that enhances a sealed condition withinthe pressure chamber 34 when the pressure chamber 34 is pressurized.When the pressure chamber 34 is sealed in this manner, the length of thepressure chamber 34 is approximately 24-30 inches and the diameter ofthe pressure chamber 34 is approximately 12 inches. The pressure chamber34 is substantially surrounded by an annular wall 40 which is operableto define the pressure chamber 34, and has a thickness which issufficient to contain the pressure within the pressure chamber 34.

The cold isostatic press 32 further includes a pressure inlet line 42and a pressure relief line 44. The pressure inlet line 42 and pressurerelief line 44 are preferably tubular passageways each regulated by apressure control mechanism (not shown) that are operable to accommodatea pressurized transfer of a gas or liquid fluid from an external source(not shown) into and out of the pressure chamber 34.

The cold isostatic press 32 is preferably designed to operate atpressures capable of compacting the powder to about 60-80% of itsdesired final density, with the preferable range being between 65-75%.The cold isostatic press 32 may be that which is available from NationalForge, Andover, Mass., or Models IP6-24-60 and IP8-36-60 which areavailable from ABB Autoclave Systems, Inc. of Columbus, Ohio. However,other suitable cold isostatic presses may be used.

The hot isostatic press 46 will now be described with reference to FIG.3. The hot isostatic press 46 is shown to include a pressure chamber 48which is defined in part by an annular wall 50, the thickness of whichis between about 6 inches and about 3 inches. In addition, the pressurechamber 48 is about 18 inches in diameter and is about 53 inches inlength. The hot isostatic press 46 further includes a lower closure 52and upper closure 54 which are threadedly attached to the annular wall50 by matching buttress threads 56 and 58. It is to be understood,however, that a pin locking mechanism may also be employed for securingthe lower closure 52 and upper closure 54 to the annular wall 50. Thelower closure 52 and upper closure 54 are operable to maintain a heatedand pressurized condition within the pressure chamber 48 during the hotisostatic pressure treatment described below.

The hot isostatic press 46 further includes a plurality of heatingelements 60 that are operable to generate thermal energy within thepressure chamber 48. Alternatively, the hot isostatic press 46 mayinclude another heating means, such as a solution jacket adjacent to thepressure chamber 48, that is operable to contain a hot fluid forproviding thermal energy to the pressure chamber 48. The hot isostaticpress 46 is also shown to include a cooling jacket 62 which comprises aplurality of coils encircling the annular wall 50. The cooling jacket 62is operable to contain a suitable heat transfer fluid for removingthermal energy from the hot isostatic press 46 by a transfer of thermalenergy into the cooling fluid. It will be understood, however, that thecooling function accomplished by the cooling jacket 62 can be performedby another cooling means disposed at a different location within the hotisostatic press 46, such as within the pressure chamber 48 or betweenthe pressure chamber 48 and the annular wall 50.

The hot isostatic press 46 further includes a heat shield 64 which islocated between the annular wall 50 and the heating elements 60. Theheat shield 64 is operable to limit heat losses from within the pressurechamber 48 and to assist in controlling the temperature within thepressure chamber 48. The hot isostatic press 46 also includes a pressuresystem (not shown) of a type well-known to those skilled in the art thatis operable to pressurize the pressure chamber 48. The pressure systemalso communicates with the pressure chamber 48 by means of a pressureinput/output line 68 that is connected to an inert gas source andcompressor of a type well-known to those skilled in the art. In apreferred embodiment, the inert gas is argon, though nitrogen, heliumand neon gases may also be used.

The hot isostatic press 46 further includes a power distribution system(not shown) of a type well-known to those skilled in the art. The powerdistribution system is used for controlling the heat and pressure withinthe pressure chamber 48. In addition, the electrical energy required bythe heating elements 60 is provided in this embodiment by an electricalpower line 72 that is connected to an electrical power source (notshown).

The hot isostatic press 46 is operable to change the temperature withinthe pressure chamber 48 from an initial room temperature of from about60° F. to about 70° F. to an operating temperature of from about 365° F.to about 420° F. In addition, the hot isostatic press 46 is alsooperable to change the pressure within the pressure chamber 48 fromapproximately atmospheric pressure to an operating pressure ofpreferably from about 7,500 pounds per square inch to about 10,000pounds per square inch. The hot isostatic press 46 may be one of severalwell-known to those skilled in the art, such as Model HP6-30, availablefrom Iso-Spectrum, Inc. of Columbus, Ohio. Other suitable hot isostaticpresses are available from National Forge of Andover, Mass. However,other suitable hot isostatic presses may be used.

The method of the preferred embodiment of the present invention will nowbe described with reference to FIG. 4 which comprises the steps 80through 96. At step 80, an ultra-high molecular weight polyethylenepowder is introduced into a first container 98 (see FIG. 3) so as tosubstantially fill the first container 98. The first container 98 ispreferably both flexible and collapsible, and is made from a materialthat has sufficient strength to contain the powder over the operatingpressure ranges during the cold isostatic pressure treatment withoutexhibiting any physical deterioration, chemical degradation or chemicalinteraction with the powder disposed therein. It is also preferred thatthe first container 98 be made of a material that will not adhere to thepowder at any time during the cold isostatic pressure treatment.

In a preferred embodiment, the first container 98 is a cylindricalpolyurethane container of dimensions approximately 6 inches in diameter,18 inches in length, and has a wall thickness of approximately one-halfinch to three-fourths inch. The first container 98 is sealed by means ofa plug 100 that is inserted into a matching port 102 at one end of thefirst container 98. The plug 100 is secured to the first container 98 bymeans of an adhesive, such as a hot melt glue, located on the top of theinterface of the plug 100 and the matching port 102. Because it isdesirable that the first container 98 be substantially evacuated priorto the cold isostatic pressure treatment, the plug 100 of the firstcontainer 98 preferably includes an evacuation/de-airing tube 104. Theevacuation/de-airing tube 104 is operable to be connected to anevacuation pump (not shown) and subsequently sealed by any suitablemeans prior to the cold isostatic pressure treatment.

It will be noted that the size and shape of the first container 98 willvary depending upon the desired size and shape of the consolidated stockbeing formed. It will also be noted that other suitable materials may beused to form the first container 98 and that other suitable means may beused for substantially sealing and evacuating the first container 98.For example, a flexible and collapsible rubber material may be employedfor constructing the first container 98. When constructed ofpolyurethane, the first container 98 may be reused provided it is notsubjected to extended periods of high temperature.

Once the first container 98 has been filled with powder, the firstcontainer 98 is sealed in the manner described above. The firstcontainer 98 is then substantially evacuated and then theevacuation/de-airing tube 104 is sealed by any suitable means such as bya clamp 106. As is illustrated by the step 82, the first container 98 isthen located within the pressure chamber 34 of the cold isostatic press32. The pressure chamber 34 is substantially sealed at step 84 toenclose the first container 98 by threading the upper cover 36 onto thematching threads 38 disposed upon the annular wall 40.

The first container 98 is then subjected to a cold isostatic pressuretreatment as indicated by the step 86 during which a uniform pressure isapplied to the first container 98. In this regard, the pressure appliedto the first container 98 is developed by introducing a pressurizedfluid into the pressure chamber 34. This pressurized fluid may be water,mineral oil or other oils having similar compressive properties, as wellas inert gases such as argon, nitrogen, helium and neon. In addition,the pressure chamber 34 may be partially filled with water while thepressurized gas may be used to fill the remainder of the pressurechamber 34. The pressure within the pressure chamber 34 is preferablyincreased as quickly as possible from approximately atmospheric pressureto a pressure sufficient to form the powder into an incompletelyconsolidated stock that can be manipulable for further processingwithout substantial degradation. Suitable maximum pressures range from1100 psi to 10,000 psi which are generally sufficient to compact thepowder to 60-80% of its final density. Below this range the incompletelyconsolidated stock is structurally unstable and above this range gasesmay become trapped within the incompletely consolidated stock duringevacuation of the first container 98. In a preferred embodiment, themaximum pressure applied to the first container 98 is approximately 1500psi, and the typical length of time for increasing the pressure to thislevel may be approximately 2 to 5 minutes. However, maximum pressureapplied to the first container 98 is dependent upon several factorsincluding the size of the first container 98, the amount of powderwithin the first container 98, the size of the resulting stock needed tomanufacture the tibial bearing 28 and the size of the pressure chamber34. The pressure is preferably held at the maximum pressure forapproximately one minute, though longer times can be used.

After the maximum pressure within the cold isostatic press 32 ismaintained for approximately one minute, the pressure is slowly reducedso as to allow the resulting incompletely consolidated stock to relaxwithin the first container 98 without yielding to outward internalpressure which can cause the incompletely consolidated stock to loseintegrity. The pressure is preferably released over a period of fromapproximately 10 to approximately 30 minutes, although longer times canbe used.

The cold isostatic press treatment enhances a uniform density within theincompletely consolidated stock and reduces internal stresses fromappearing within the material being formed during the subsequent hotisostatic pressure treatment. In addition, the shape of the incompletelyconsolidated stock is in large part dependent upon the shape of thefirst container 98. The incompletely consolidated stock resulting fromthe cold isostatic press treatment is typically compacted to a preferreddensity of about 70% of its desired final density following the hotisostatic pressure treatment.

After the incompletely consolidated stock has been removed from thefirst container 98, the incompletely consolidated stock is placed in asecond container 108 (see FIG. 5) as indicated by the step 88. Thesecond container 108 is preferably a collapsible container made from amaterial that has sufficient strength to contain the incompletelyconsolidated stock over the temperature and pressure ranges encounteredin the hot isostatic press treatment without exhibiting any physicaldeterioration, chemical degradation or chemical interaction with theincompletely consolidated stock. It is also preferred that the secondcontainer 108 be made of a material that will not adhere to theincompletely consolidated stock at any time during the hot isostaticpressure treatment. In a preferred embodiment, the second container 108is a foilized heat sealable bag that has an external surface formed froma layer of an aluminum foil with a polyester vapor barrier, and has aninternal surface formed from a heat-sealable, low density polyethylenelayer on its internal surface. The second container 108 may typically beapproximately 18 inches in length, approximately 12 inches in width andhave a wall thickness of between approximately 2-3 mils. As will beappreciated by those skilled in the art, the second container 108 may bemade from other suitable materials as well.

Because it is desirable to have the second container 108 besubstantially evacuated prior to the hot isostatic pressure treatment,the second container 108 preferably includes an evacuation tube 110 thatis operable to be connected to a vacuum pump (not shown). In thisregard, the evacuation tube 110 is placed in the second container 108and then a heat sealer is used to seal that region of the secondcontainer 108 which is not immediately adjacent to the evacuation tube110. Hot melt glue is then placed around the region of the secondcontainer 108 which is adjacent to the evacuation tube 110.

It will be noted that the size and shape of the second container 108will vary depending upon the desired size and shape of the consolidatedstock being formed. It will also be noted that other suitable materialsmay be used for the second container 108 and that other suitable meansmay be used for sealing and evacuating the second container 108.

After the incompletely consolidated stock is placed in the secondcontainer 108, the second container 108 is evacuated in similar fashionto the evacuation of the first container 98 as indicated by the step 90.A heat sealer is then used to substantially enclose the second container108 at a region below the evacuation tube 110. The evacuation tube 110may then be removed from the second container 108.

Once the incompletely consolidated stock is placed within the secondcontainer 108 and the second container 108 is sealed and evacuated. Thesecond container 108 is then placed into the pressure chamber 48 of thehot isostatic press 46 as indicated by the step 92 of the presentinvention. The lower closure 52 and the upper closure 54 of the hotisostatic press 46 are then closed to substantially enclose the secondcontainer 108 within the hot isostatic press 46.

At step 94, the incompletely consolidated stock undergoes the hotisostatic pressure treatment. In this regard, the pressure within thepressure chamber 48 is initially raised to approximately 24 psi whilethe temperature of the hot isostatic press 46 is raised between 365° F.and 420° F. Below this range the incompletely consolidate stock does notmelt and above this range the polyethylene may degrade. Preferably, thetemperature of the hot isostatic press is raised to between 365°-385° F.to minimize the possibility that degradation will occur. Mostpreferably, the temperature is raised to 365° F. Once 365° C. isreached, the temperature of the hot isostatic press 46 is raised asquickly as possible and may typically heat between one to three hours.

When the temperature of the hot isostatic press 46 reaches approximately365° C., the pressure within the pressure vessel 46 is also increasedover a 1-2 hours period to a pressure preferably between about 7,500 toabout 10,000 psi. It will be appreciated that the maximum pressure mayrange from about 3,000 psi to about 40,000 psi. However, pressures below3,000 psi or above 40,000 psi tend to cause consolidation errors tooccur or may cause the resulting completely consolidated stock to havean undesirable crystalline structure. The preferred maximum pressurebetween 7,500 psi and 10,000 psi is dependent upon several factorsincluding the size, shape and construction of the second container, thedimensions of the pressure chamber 48 and the desired final diameter ofthe resulting completely consolidated stock. In addition, the durationof the hot isostatic pressure treatment may also depend on the size ofthe resulting completely consolidated stock. For example, smallerdiameters of the completely consolidated stock (e.g., 11/2 inches)typically require less time to become fully compacted, while largerdiameters of completely consolidated stock, such as 4 inches, typicallyrequire more time to become fully cured. In addition, the use of thelowest satisfactory pressure is desirable as it would tend to prolongequipment life. An inert gas such as argon is preferably used in the hotisostatic press 46 as the pressure medium. Alternative selections forthe pressure medium include nitrogen, helium and neon gases, althoughthese gases can be chemically reactive under certain conditions.

Once the temperature and pressure have reached the desired levels, thetemperature and pressure of the pressure chamber 48 remains relativelyconstant for a given dwell time. During this dwell time, the powder isfurther compressed so as to minimize any compression release that mayoccur following termination of the application of heat and pressure.Preferred dwell times are dependent upon the desired final diameter ofthe consolidated stock being produced, and range from approximately 45minutes to several hours or more. For example, typical desired dwelltimes may be approximately 45 minutes to approximately 1 hour for a 1inch diameter consolidated stock, approximately 2 hours for a 21/2 inchdiameter consolidated stock, and approximately 5 hours for a 4 inchdiameter consolidated stock.

After the second container 108 has been subjected to the desiredtemperature and pressure for the given dwell time, the hot isostaticpress 46 is allowed to cool to room temperature. After the temperatureof the hot isostatic press 46 cools to approximately 100° F., thepressure within the pressure chamber 48 is gradually decreased toapproximately atmospheric pressure over a period of time that isdependent upon the desired final diameter of the consolidated stockbeing produced. In this regard, the pressure for larger diameters ofconsolidated stock may be reduced more slowly because they may typicallyhave a larger internal compression and larger potential energy that aremore likely to release upon removal of pressure. For example, a 1 hourpressure release time is preferred for a 4 inch diameter consolidatedstock, while a 20 minute pressure release time may be sufficient for a11/2 inch diameter consolidated stock. After the release time haselapsed, the pressure chamber 48 is then opened and the second container108 is removed from the pressure chamber 48.

The consolidated stock is removed from the second container 108 and ismachined at step 96 under methods well-known to those skilled in the artto produce the desired product, such as the tibial bearing 28,acetabular cup replacement or other biocompatible component. Aftermachining the consolidated stock at step 92 to form the tibial bearing28, the tibial bearing 28 is then sterilized in a manner well-known tothose skilled in the art.

In addition to using of the hot isostatic press 46 in forming acompletely consolidated stock, the hot isostatic press 46 may be alsoused to enhance the adhesion between materials which form a compositebiocompatible component. For example, as shown in FIG. 7, abiocompatible component 112 representing a femoral hip stem of a hipjoint prosthesis is shown. The biocompatible component 112 is preferablyformed of a biocompatible thermoplastic having a biocompatible fibrousmaterial disposed therein. The biocompatible thermoplastic may bepolysulfone, poly ether ether ketone (PEEK), or poly aryl ether ketone(PAEK), though other suitable materials may be used. The amount andorientation of the biocompatible fibrous material within thebiocompatible component 112 is selected to achieve the desiredstructural modulus for the biocompatible component. The biocompatiblefibrous material may be either continuous or chopped fibers, thoughother suitable materials may be used.

When used in this manner, a sheet of the biocompatible thermoplasticsuch as polysulfone is first formed into two portions, each portionhaving a shape generally corresponding to one-half of the biocompatiblecomponent 112. Each portion of the biocompatible thermoplastic is thenplaced within the container 114 with the biocompatible fibrous materialdisposed between the portions. The container 114 is preferably made fromcopper or stainless steel. However, other suitable materials such ashigh temperature silicon, which does adhere to the polysulfone, may alsobe used. The container 114 is then filled with zirconium oxide beads(i.e., Zr₂ O₃) and is then evacuated using the evacuation tube 115 whichis then sealed. It will be appreciated that zirconium oxide beads do nothave to be used when the container 114 is made from a very pliablematerial such as high temperature silicon. The container 114 is thenplaced in the hot isostatic press 46 and is subjected to the hotisostatic pressure treatment in a manner similar to that describedabove. In this regard, the maximum temperature of the isostatic pressuretreatment is preferably slightly above the melting temperature of thebiocompatible thermoplastic. In addition, the pressure applied and theduration of the hot isostatic pressure treatment should be sufficient tocause the biocompatible thermoplastic to encapsulate the biocompatiblefibrous material. Preferably, the temperature will fall within the rangeof 400-440° F. while the pressure will be greater than between 5000 psiand 7500 psi, and most preferably greater than 7500 psi. It will beunderstood by those skilled in the art, however, that the temperature,pressure and duration of the hot isostatic pressure treatment willdepend upon the specific materials being used.

The isostatic press 46 may also be used to reduce the voids in a stockof biocompatible composite material prior to being machined into abiocompatible component. For example, as shown in FIG. 8, abiocompatible material stock 116 is shown as being disposed within acontainer 118. The biocompatible material stock 116 may be made frompolysulfone, poly ether ether ketone (PEEK), or poly arly ether ketone(PAEK), though other suitable materials may be used. The container 118may be a stainless steel or a copper container. However, the containermay also be a stainless steel heat treat bag of the type which isavailable from Sentry Company, Foxboro, Mass., other suitable containersmay be used.

When used in this manner, the biocompatible material stock 116 is firstplaced within the container 118 and then the container 118 is filledwith zirconium oxide beads. It will be appreciated, however, thatzirconium oxide beads do not have to be used if the container 118 ismade from a pliable material such as a heat treat bag. The container 118is then sealed. A vacuum is then drawn on the container 118 through theevacuation tube 119 and then the evacuation tube 119 is then sealed byclosing a valve connected to the evacuation tube 119. The container 118,with the biocompatible stock material 116 located therein, is placed inthe isostatic press 46 and the temperature and pressure of the isostaticpress 46 are raised to such an extent that the voids formed within thebiocompatible stock material 116 are reduced. This reduction in voidsoccurs because of the external pressure applied to the exterior of thecontainer 118. By using the isostatic press 46 in this manner, theresulting biocompatible material has improved consolidation. Thetemperature to which the hot isostatic press 46 is raised will besubstantially that of the glass transition temperature of the resin ofthe biocompatible stock material 116, while the pressure applied by thehot isostatic press 46 is as high as reasonably possible. Preferably,the temperature will fall within the range of 400°-440° F. while thepressure will be greater than between 5000 psi and 7500 psi, and mostpreferably greater than 7500 psi. However, other suitable temperaturesand pressures may be used.

The isostatic press 46 may also be used to enhance the adhesion of aporous coating on a biocompatible component formed from a compositematerial. As shown in FIG. 9, the biocompatible component 120 includes aporous coated surface 122 which is used to facilitate adhesion of thebiocompatible component immediately after surgery. The porous coatedsurface 122 may be applied by a plasma spray operation and may comprisean alloy of Ti-6Al-4V, commercially pure titanium, a cobalt chrome alloyor other biocompatible materials.

When the isostatic press 46 is used to enhance adhesion of a porouscoating 122 onto the biocompatible component 120, the porous coatedsurface 122 is first applied to the biocompatible component 120 by aplasma spray operation. The biocompatible component 120 is then placedin a container 124 and then the container 124 is filled with zirconiumoxide beads. The container 124 is then sealed and then is evacuatedthrough the evacuation tube 125. The container 124 is preferably made ofstainless steel or copper. However, other suitable materials such ashigh temperature silicon, which does not adhere to the component 120 mayalso be used. It will be appreciated, however, that zirconium oxidebeads do not have to be used if the container 124 is made from a pliablematerial such as high temperature silicon. The container 124, with thebiocompatible component 120 inside, is then placed in a hot isostaticpress 46 which is then operated in a manner similar to that describedabove. Preferably, the temperature will fall within the range of400°-440° F. while the pressure will be greater than between 5000 psiand 7500 psi, and most preferably greater than 7500 psi. As a result,the adhesion of the porous coated surface 122 to the biocompatiblecomponent 120 is improved.

The hot isostatic press 46 may also be used to enhance adhesion ofporous coated pads on a biocompatible component. As shown in FIG. 10,the biocompatible component 126 includes a plurality of porous coatedpads 128 which are used to facilitate fixation of the biocompatiblecomponent 126 immediately after surgery. The porous coated pads 128 maybe made from a titanium alloy such as Ti-6Al-4V, commercially puretitanium, a cobalt chrome alloy or other biocompatible metal alloys.While the porous coating on the porous coated pads 128 may be applied bya flame spray, plasma spray or sputtering techniques, it will beappreciated that other suitable methods may be used.

When the hot isostatic press 46 are used to enhance adhesion of theporous coated pads 128 onto the biocompatible component 126, the regionson the biocompatible component 126 where the porous coated pads 128 areto be placed are first coated with methylene chloride to partiallydissolve those regions at the biocompatible component 126. The porouscoated pads 128 are then applied to the biocompatible component 126 andare temporarily secured thereto. The biocompatible component 126,together with the porous coated pads 128, are then placed in thecontainer 130 and then the container 130 is filled with zirconium oxidebeads. The container may be made from stainless steel or copper, thoughother suitable materials may be used. In this regard, pliable materialssuch as high temperature silicon which is able to withstand theoperating temperatures and pressures may be used which do notnecessarily require the use of zirconium oxide beads. After thecontainer 130 is evacuated through the evacuation tube 132 and theevacuation tube 132 is sealed, the container 130 is placed in the hotisostatic press 46 which is then operated in a manner similar to thatdescribed above. Preferably, the temperature will fall within the rangeof 400°-440° F. while the pressure will be greater than between 5000 psiand 7500 psi, and most preferably greater than 7500 psi. As a result,the porous coated pads 128 are relatively securely attached to thebiocompatible component 126.

The principles of the present invention described broadly above will nowbe described with reference to the following specific example, withoutintending to restrict the scope of the present invention.

EXAMPLE 1

Polyethylene powder having a molecular weight of approximately 3 millionand conforming to ASTM F 648-84 is introduced into a first containerformed from polyurethane of approximately 6 inches in diameter, 18inches in length, and of approximately one-half inch wall thickness. Thefirst container is substantially sealed and substantially evacuated, andis then placed in the cold isostatic press 32. The cold isostatic pressis then closed, and the pressure therein is increased by introducingpressurized water into the pressure vessel to approximately 1500 psiover approximately a 2 minute period. The pressure is then maintainedwithin the cold isostatic press for approximately 1 minute, and is thendecreased to atmospheric pressure over approximately a 10 minute period.The cold isostatic press is then opened and the first container isremoved and opened to reveal a cylindrical incompletely consolidatedstock of dimensions 51/2 inches in diameter and 14 inches in length.

The incompletely consolidated stock is then placed in a secondcontainer. The second container is formed from an aluminum foil layerwith a polyester vapor barrier on its external surface and aheat-sealable, low density polyethylene layer on its internal surface.In addition, the second container is approximately 18 inches in length,12 inches in width and has a wall thickness of 2-3 mils. The secondcontainer is then evacuated and sealed. The second container is thenplaced in the hot isostatic press and then the hot isostatic press isclosed. The temperature of the hot isostatic press is then increasedfrom room temperature to 400° F. over approximately a 1 hour period. Atthe same time, pressurized argon gas is introduced into the pressurechamber of the hot isostatic press to increase the internal pressurefrom atmospheric pressure to 10,000 pounds per square inch overapproximately a 3 hour period. After reaching 400° F. and 10,000 psi,the temperature and pressure within the hot isostatic press aremaintained for approximately 5 hours. The temperature is then allowed todecrease to 100° F. over approximately a 4 hour period. At the sametime, the pressure within the hot isostatic press is decreased toatmospheric pressure over a 30 minute period. After release of thepressure and cooling to room temperature, the hot isostatic press isopened and the second container is removed. The second container is thenopened to reveal a relatively completely consolidated stock whichresembles a cylinder having dimensions of 4 inches in diameter and 12inches in length. The relatively completely consolidated stock is testedto have a crystallinity of 45%-60%. The consolidated stock issubsequently machined to form a tibial bearing which is then packagedand then exposed 2.5 megarads of radiation from a cobalt source forsterilization.

EXAMPLE 2

Two portions of polysulfone approximately 5 mils in thickness are vacuumformed into a shape which resembles each half of a biocompatiblecomponent in the form of a hip joint prosthesis described above. Eachportion of the polysulfone representing the halves of the biocompatiblecomponent is then placed in a stainless steel container with continuouscarbon fibers disposed between the portions. The container is placed inthe hot isostatic press and then the hot isostatic press is closed. Thetemperature of the hot isostatic press is then increased from roomtemperature to approximately 410° F. over approximately a one hourperiod. At the same time, pressurized argon gas is introduced into thepressure chamber of the hot isostatic press to increase the internalpressure from atmospheric pressure to 7500 psi over approximately athree-hour period.

After reaching 410° F. and 7500 psi, the temperature and pressure withinthe hot isostatic press are maintained for approximately a thirty minuteperiod. The temperature is then allowed to decrease to room temperatureafter which the pressure is released. After the pressure has beenreleased and the hot isostatic press has been cooled to roomtemperature, the hot isostatic press is opened and the container isremoved. The container is then opened to reveal a biocompatiblecomponent in which the polysulfone has encapsulated the carbon fibers.

EXAMPLE 3

A biocompatible component in the form of a hip joint prosthesisdescribed above is initially formed from continuous carbon fibers whichare encapsulated by polysulfone. Methylene chloride is applied to theupper portion of the biocompatible component so as to partially dissolvethe polysulfone. After portions of the polysulfone have been melted, aplurality of porous coated pads described above are applied to theregion of the biocompatible component to which methylene chloride isapplied. The biocompatible component with the porous coated padsattached is then placed in a stainless steel container of the typedescribed above which is then filled with zirconium oxide beads. Afterthe container is evacuated and sealed, the container is placed in a hotisostatic press and then the hot isostatic press is closed.

The temperature of the hot isostatic press is then increased from roomtemperature to approximately 410° F. over approximately a one hourperiod. At the same time, pressurized argon gas is introduced into thepressure chamber of the hot isostatic press to increase the internalpressure from atmospheric pressure to 5000 psi over an approximately aone hour period. After reaching 410° F. and 5000 psi, the temperature inthe pressure of hot isostatic press are maintained for approximately 45minutes. The pressure is then allowed to decrease over approximately afour-hour period, while the temperature within the hot isostatic pressis decreased to ambient over a thirty minute period. After release ofthe pressure and cooling to room temperature, the hot isostatic press isopen and the container is removed. The second container is then openedto reveal the biocompatible component with the porous coated padsrelatively securely attached.

EXAMPLE 4

A biocompatible material stock in the form of a slab is placed within astainless steel container which is then formed with zirconium oxidebeads. The slab is approximately 1.5 inches thick and is 10 in lengthand 10 in width. After the container is then evacuated and sealed in themanner described above, the container together with the biocompatiblematerial stock is placed in a hot isostatic press. The temperature ofthe hot isostatic press is then increased from room temperature toapproximately 430° F. over approximately a 0.75 hour time period. At thesame time, pressurized argon gas is introduced into the hot isostaticpress to increase the internal pressure from atmospheric toapproximately 7500 psi over approximately a one hour time period.

After reaching 430° F. and 7500 psi, the temperature and pressure of theisostatic press are maintained for approximately 0.75 hours. Thetemperature is then reduced to room temperature after which the pressureis released. After pressure has been released and the hot isostaticpress has been cooled to room temperature, the hot isostatic press isopened and the container is removed. The container is then opened toreveal a slab of biocompatible material stock in which the voids arereduced.

EXAMPLE 5

A biocompatible component in the form of a stem of a hip jointprosthesis described above is initially formed from carbon fibers whichare encapsulated by a polysulfone. The biocompatible component is thenultrasonic cleaned in water to remove surface containments. Thebiocompatible component is then subjected to a grit blasting operation.In this regard, the portions of the biocompatible component which arenot to receive the porous coating are initially covered with polyvinylchloride tape. The biocompatible component is then placed in front ofgrit blaster operating at 40 psi with a 1/2 inch nozzle using a 16 gritsilicon carbide particles. By grit blasting the biocompatible component,a roughen surface is formed in the region which is exposed to theparticles. The polyvinyl chloride tape is then removed from thebiocompatible component.

It will be appreciated that other means may be used for obtaining aroughened surface on the biocompatible component. For example, a 1/16"drill bit may be used to form the roughened surface. In this regard, thedrill bit may be randomly disposed against the portion of thebiocompatible component where the roughened surface is to be formedthereby generating a plurality of holes. This process is continued untilthe desired surface roughness is obtained. A dove-tail cutter may alsobe used to form the roughened surface. In this regard, a dove-tailcutter having a cutting surface of a maximum of 1/32" in diameter may bedisposed against the surface of the biocompatible component which is tobe roughened and then the axis of rotation of the cutter is moved in acircular fashion. The dove-tail cutter is then removed from thebiocompatible component and then disposed against another portion of thesurface of the biocompatible component. This process is repeated untilthe desired surface roughness is obtained.

The biocompatible component is then cleaned in a second ultrasoniccleaning operation. The portions of the biocompatible component whichare not to receive the porous coating are then covered with heat tapewhich is resistant to the high temperature which are to be generatedduring the plasma spray operation. Care is taken so that the portions ofthe biocompatible component which are covered by the porous coating arenot physically contacted after the second ultrasonic cleaning operationuntil the porous coating is applied. In this regard, the storage rackwhich is used to transport the biocompatible component to the plasmaspray chamber supports the biocompatible component only at those areaswhich are not to be coated with porous coating.

The biocompatible component is then placed in a plasma spray chamber insuch a manner that portions of the biocompatible component whichreceives the porous coating is not touched. The spray chamber isinitially evacuated to approximately 10 millitorr and then backfilledwith argon gas to a pressure slightly above atmospheric. The plasmaspray is then applied to the biocompatible component for a period ofapproximately 20 seconds. After the plasma spray has been applied forthis period of time, the biocompatible component is placed in front ofthe exhaust fan of the spray chamber for approximately 1 minute. Thisprocess of applying the plasma spray and then cooling the biocompatiblecomponent is repeated approximately 8 times. The biocompatible componentis then removed from the spray chamber and then sandpaper is used toremove loose material from the biocompatible component. Thebiocompatible component is washed in a water jet operating at 900 psi toremove additional residue.

The biocompatible component is then placed in a stainless steelcontainer of the type described above which is then filled withzirconium oxide beads. The container is then evacuated and sealed. Thecontainer, together with the biocompatible component, is then placed ina hot isostatic press and then the hot isostatic press is closed. Thetemperature of the hot isostatic prees is then increased from roomtemperature to approximately 430° F. and while the pressure is increasedto 7500 psi. The temperature and pressure of the hot isostatic press aremaintained for approximately 45 minutes. The temperature of the hotisostatic press is then allowed to cool to approximately 100° F. over aone hour time period and then the pressure of the hot isostatic press isdecreased to atmospheric over a thirty-minute time period. The hotisostatic press is then cooled to room temperature after which the hotisostatic press and the container is removed. The container is thenopened to reveal a biocompatible component with the porous coatedsurface relatively securely attached.

It will be appreciated that the foregoing description of the preferredembodiment of the invention is presented by way of illustration only andnot by way of any limitation. For example, a preheating step may be usedto preheat the incompletely consolidated stock prior to placing theconsolidated stock in the hot isostatic press. In addition, the variouscontainers may be of different sizes and of different materials.Furthermore, filters such as paper towels may be located within thevarious evacuation tubes. Various alternatives and modifications may bemade to the illustrative embodiment without departing from the spiritand scope of the invention.

What is claimed is:
 1. A method for molding a powder to form asubstantially completely consolidated stock of material, said methodcomprising the steps of:forming an incompletely consolidated stock fromsaid powder; and forming said substantially completely consolidatedstock from said incompletely consolidated stock; wherein said step offorming an incompletely consolidated stock comprises the stepsof:placing said powder into a first flexible container; and subjectingsaid first flexible container to a cold isostatic pressure treatment soas to form said incompletely consolidated stock.
 2. The method accordingto claim 1, wherein said step of placing said powder into a firstflexible container includes the steps of:placing said powder into asubstantially cylindrical container formed from polyurethane; enclosingsaid substantially cylindrical container with a plug; and evacuating andthen sealing said substantially cylindrical container.
 3. The methodaccording to claim 1, wherein said step of forming said substantiallycompletely consolidated stock from said incompletely consolidated stockincludes the steps of:placing said incompletely consolidated stock intoa second flexible container; and subjecting said second flexiblecontainer to a hot isostatic pressure treatment so as to form saidsubstantially completely consolidated stock.
 4. The method according toclaim 3, wherein said step of placing said incompletely consolidatedstock into a second flexible container includes the step of forming saidsecond flexible container from a layer of aluminum foil, a polyestervapor barrier, and a heat sealable layer.
 5. The method according toclaim 3, wherein said step of placing said incompletely consolidatedstock into a second flexible container includes the stepsof:substantially enclosing said second flexible container; andsubstantially evacuating said second flexible container.
 6. A method formolding a powder to form substantially completely consolidated stock ofmaterial, said method comprising the steps of:placing said powder into afirst flexible container; substantially evacuating and sealing saidfirst flexible container; subjecting said first flexible container to afirst pressure treatment so as to form an incompletely consolidatedstock from said powder; placing said incompletely consolidated stockinto a second flexible container; substantially evacuating said secondflexible container; and subjecting said second flexible container to asecond pressure treatment so as to form said substantially completelyconsolidated stock from said incompletely consolidated stock.
 7. Themethod according to claim 6, wherein said step of placing said powderinto a first flexible container includes the step of placing said powderinto a substantially cylindrical container formed from polyurethane. 8.The method according to claim 7, wherein said step of substantiallyevacuating and sealing said first container includes the stepsof:placing a plug with a first tube disposed therein into saidsubstantially cylindrical container; evacuating said substantiallycylindrical container through said first tube; and sealing said firsttube after said substantially cylindrical container has been evacuated.9. The method according to claim 8, wherein said step of placing saidincompletely consolidated stock into a second flexible containerincludes the step of forming said second flexible container from a layerof aluminum foil, a polyester vapor barrier, and a heat sealable layer.10. The method according to claim 6, wherein said step of subjectingsaid second flexible container to a second pressure treatment includesthe step of subjecting said second flexible container to a hot isostaticpressure treatment so as to form said substantially completelyconsolidated stock.
 11. A method for molding a powder to form asubstantially consolidated stock of material, said method comprising thesteps of:preparing said substantially consolidated stock from a powderby: (a) subjecting said powder to a cold isostatic pressure treatmentthereby forming an incompletely consolidated stock from said powder, (b)subjecting said incompletely consolidated stock to a hot isostaticpressure treatment to form said substantially consolidated stock fromsaid incompletely consolidated stock while impeding the formation of acrystalline structure; and machining said substantially consolidatedstock to form said biocompatible component.
 12. A method for molding apowder to form a substantially consolidated stock of material accordingto claim 11, wherein said powder is an ultra high molecular weightpolyethylene powder.
 13. The method for molding a powder to form asubstantially consolidated stock of material according to claim 11,wherein said step of subjecting said powder to a cold isostatic pressuretreatment includes the steps of:placing said powder into a firstflexible container; and subjecting said first flexible container to apressure sufficient to form said incompletely consolidated stock. 14.The method for molding a powder to form a substantially consolidatedstock of material according to claim 13, wherein said step of placingsaid powder into a first flexible container includes the stepsof:placing said powder in a substantially cylindrical container formedfrom polyurethane; enclosing said substantially cylindrical containerwith a plug; and evacuating and sealing said substantially cylindricalcontainer.
 15. The method for molding a powder to form a substantiallyconsolidated stock of material according to claim 11, wherein said stepof subjecting said incompletely consolidated stock to a hot isostaticpressure treatment includes the steps of:placing said incompletelyconsolidated stock into a second flexible container; and subjecting saidsecond flexible container to a sufficient pressure and temperature so asto form said substantially completely consolidated stock.
 16. The methodfor molding a powder to form a substantially consolidated stock ofmaterial according to claim 15, wherein said step of placing saidincompletely consolidated stock into a second flexible containerincludes the step of placing said incompletely consolidated stock in afoilized heat sealable container formed from a layer of aluminum foil, apolyester vapor barrier, and a heat sealable layer.
 17. The method formolding a powder to form a substantially consolidated stock of materialaccording to claim 18, wherein said step of placing said incompletelyconsolidated stock into a second flexible container includes the stepsof:substantially enclosing said second flexible container; andsubstantially evacuating said second flexible container.
 18. A methodfor molding a powder to form a substantially consolidated stock ofmaterial, said method comprising the steps of:preparing saidsubstantially consolidated stock from a powder by: (a) subjecting saidpowder to a cold isostatic pressure treatment thereby forming anincompletely consolidated stock from said powder, (b) subjecting saidincompletely consolidated stock to a hot isostatic pressure treatment toform said substantially consolidated stock from said incompletelyconsolidated stock while controlling crystallinity so as tosubstantially avoid increasing the crystallinity of the substantiallyconsolidated stock above that of the powder; and machining saidsubstantially consolidated stock to form said biocompatible component.19. A method for molding a powder to form a substantially consolidatedstock of material according to claim 18, wherein said powder is an ultrahigh molecular weight polyethylene powder.